The present invention relates generally to improved blast nozzles for removing adherent material such as paint, scale, dirt, grease and the like from solid surfaces with abrasive particles propelled by air. In particular, the present invention is directed to a novel blast nozzle having a specified shape and dimensions to improve blast-cleaning efficiency.
In order to clean a solid surface so that such surface can again be coated such as, for example, to preserve metal against deterioration, or simply to degrease a solid surface such as surfaces contacting food or building structures which contain food serving or food processing operations, it has become common-practice to use an abrasive blasting technique wherein abrasive particles are propelled by a high pressure fluid against the solid surface in order to dislodge previously applied coatings, scale, dirt, grease or other contaminants. Various abrasive blasting techniques have been utilized to remove the coatings, grease and the like from solid surfaces. Thus, blasting techniques comprising dry blasting which involves directing the abrasive particles to a surface by means of pressurized air typically ranging from 30 to 150 psi, wet blasting in which the abrasive blast media is directed to the surface by a highly pressurized stream of water typically 3,000 psi and above, multi-step processes comprising dry or wet blasting and a mechanical technique such as sanding, chipping, etc. and a single step process in which both air and water are utilized either in combination at high pressures to propel the abrasive blast media to the surface as disclosed in U.S. Pat. No. 4,817,342, or in combination with relatively low pressure water used as a dust control agent or to control substrate damage have been used.
A typical dry blasting apparatus as well as a wet blasting apparatus which utilizes highly pressurized air to entrain, carry and direct the abrasive blast media to the solid surface to be treated and low pressure water for dust control comprises a dispensing portion in which the blast media typically contained in a storage tank is entrained in highly pressurized air, a flexible hose which carriers the air/blast media mixture to the blast nozzle and which allows the operator to move the blast nozzle relative to the surface to be cleaned and the blast nozzle which accelerates the abrasive blast media and directs same into contact with the surface to be treated. The blast nozzle is typically hand-held by the operator and moved relative to the targeted surface so as to direct the abrasive blast media across the entire surface to be treated.
The blast media or abrasive particle most widely used for blasting surfaces to remove adherent material is sand. Sand is a hard abrasive that is very useful in removing adherent materials such as paint, scale and other materials from metal surfaces such as steel. While sand is a most useful abrasive for each type of blasting technique, there are disadvantages in using sand as a blast media. For one, sand, i.e. silica, is friable and upon hitting a metal surface will break into minute particles that are small enough to enter the lungs. The minute silica particles pose a substantial health hazard. Additionally, much effort is needed to remove the sand from the surrounding area after completion of blasting. Still another disadvantage is the hardness of sand itself. Thus sand cannot readily be used as an abrasive to remove coatings from relatively soft metals such as aluminum or any other soft substrate such as plastic, plastic composite structures, concrete or wood, as such relatively soft substrates can be excessively damaged by the abrasiveness of sand. Moreover, sand cannot be used around moving parts of machinery inasmuch as the sand particles can enter bearing surfaces and the like.
An alternative to non-soluble blast media such as sand, in particular, for removing adherent coatings from relatively soft substrates such as softer metals such as aluminum, composite surfaces, plastics, concrete and the like is sodium bicarbonate. While sodium bicarbonate is softer than sand, it is sufficiently hard to remove coatings from aluminum surfaces and as well remove other coatings such as paint, dirt, and grease from non-metallic surfaces without harming the substrate surface. Sodium bicarbonate is not harmful to the environment and is most advantageously water-soluble such that the particles that remain subsequent to blasting can be simply washed away without yielding environmental harm.
Sodium bicarbonate blast media has been directed to the targeted surface by means of venturi-type blast nozzles typically used for directing harder abrasive media such as sand. Such blast nozzles include a hollow converging inlet portion, a venturi orifice and a diverging hollow outlet portion downstream of the orifice. Since the sodium bicarbonate blast media is less dense than sand or other hard abrasive media, the blast nozzles used to direct sand do not necessarily have the proper dimensions for accelerating the sodium bicarbonate media there through to provide the optimum velocity and most productive cleaning. It therefore, would be advantageous to design a blast nozzle which would be most useful for blast cleaning with less dense media such as sodium bicarbonate so as to obtain optimal cleaning productivity with such blast media.
It has been suggested previously that by increasing the length of the nozzle, productivity can be increased at least with respect to blasting with sand. Unfortunately, the blast nozzles used for propelling sand against a targeted surface must be formed of very heavy ceramic material to withstand the abrasive nature of the sand. Longer nozzles simply are not practical since by lengthening the nozzle, the weight of the nozzle would be greatly increased making hand-held operation of such nozzles extremely difficult. In addition, the cost would be excessive and the nozzles would be fragile and subject to breakage. Using a softer sodium bicarbonate blast media, however, allows the use of substantially lighter materials of construction to form the blast nozzle. For example, very thin stainless steel can be used to form the blast nozzle. The blast nozzle can now be lengthened without adding excessive weight thereto. Hand-held operation is now practical and a substantially improved productivity can be achieved whether dry blasting or atomized water blasting is used. The present inventor has disclosed in U.S. Pat. No. 5,484,325 that in those blast nozzles comprising a converging inlet, a venturi throat and a diverging outlet, providing the blast nozzle with a total length of at least about four times, preferably at least five times and, more preferably, at least about six times the length of the inlet, substantially improved production can be achieved by blasting with sodium bicarbonate. This improved productivity has been found whether during dry blasting or utilizing dry blasting with atomized water for dust control.
Further disclosed in aforementioned U.S. Pat. No. 5,484,325 is that optimal productivity for blast cleaning a surface with a softer, less dense blast media such as sodium bicarbonate can be achieved by a venturi-type blast nozzle characterized more specifically than by the mere relative total length to inlet length of the blast nozzle. As disclosed therein, it was found that optimal productivity can be achieved if the outlet length, that being the length of the venturi-type nozzle immediately downstream of the orifice (throat) to the outlet of the nozzle is approximately 20 times the diameter of the orifice. Generally, it was found that an outlet length that is 18 to 24 times the orifice diameter provides optimal productivity. At outlet lengths below the range cited, productivity is adversely affected. At lengths above the range, productivity is no longer improved or may be adversely affected. Along with the outlet length, optimal productivity is achieved if the outlet diameter is approximately 1.5 times the orifice diameter. Deviations of more than 10% below this parameter adversely affect productivity. Thus, the outlet diameter should be at least 1.35 times the orifice diameter. Deviations above 1.65 times the orifice diameter do not show benefits at media flow rates typically used to blast with sodium bicarbonate, i.e., 2-4 lbs./min. At higher flow rates, larger nozzle outlets may show productivity improvements.
As further disclosed in U.S. Pat. No. 5,484,325, with softer and friable blast media, passage through the converging inlet section of the venturi-type blast nozzle often degrades the particles of the media, creating particles of smaller mass and often causing turbulent flow in the inlet section thereby reducing the velocity of the particles as they travel through the blast nozzle. The loss of mass and velocity reduces the force of the particle on the targeted surface and, thus, can reduce productivity of the nozzle. Thus, the converging inlet section of a blast nozzle for directing the softer abrasive media should converge at a relatively minor angle, typically from between about 5xc2x0 to 15xc2x0 from horizontal, preferably, approximately 10xc2x0. To further eliminate turbulent flow, the diameter of the inlet should be approximately equivalent to the inside a diameter of the blast hose which supplies the blast media to the nozzle. Preferably, the inlet diameter should not deviate more than approximately 25% plus or minus from the inlet diameter of the supply hose. The longitudinal length of the orifice is optimum at lengths about equivalent to the orifice diameter. Larger orifice lengths have not been found to yield any significant improvement in productivity.
While the nozzle parameters as described above have been optimized for improving blast cleaning with a soft media such as sodium bicarbonate, the formation of blast nozzles from a hard ceramic allow such nozzles to be used for blast cleaning with harder, more dense substances, either added with the softer abrasive or as the sole abrasive agent. It is believed that the parameters described above improve productivity of blast cleaning using the harder, more dense abrasive media even though the exact ratios of nozzle length to orifice diameter, outlet diameter to orifice diameter, etc. as described above may not yield the optimum productivity with these abrasives.
As disclosed in U.S. Pat. No. 5,484,325, the parameters for improving the performance of blast nozzles as described, define nozzles having a circular cross-section (round nozzle) of specified orifice and outlet areas and angle of divergence in the outlet section. Thus, it is stated that the dimensions of a nozzle of any cross-section can be calculated based on the described ratios. No further explanation is provided, however.
A standard round nozzle comprises a converging hollow conical inlet section, a circular venturi throat and a contiguous diverging hollow conical outlet section. The standard round nozzle is highly productive inasmuch as it provides for the maximum acceleration of the abrasive particles through the nozzle relative to other nozzle shapes. This is in part due to the fact that the circular cross section yields the smallest internal nozzle surface area, thus, greatly reducing friction between the expanding air containing the abrasive media and the internal surfaces of the nozzle. Contact of the air/abrasive mix with the internal surfaces of the nozzle can result in deceleration of the abrasive particles and consequent reduction in blast cleaning effectiveness. While the circular cross section of the round nozzle yields the smallest internal surface area, the xe2x80x9chotspotxe2x80x9d, that being the area of the target surface which is contacted at one time with the media, produced by the round nozzle is rather compact. For cleaning large surface areas, the use of a round nozzle may be quite inefficient due to the reduced size of the hotspot, despite the fact that the abrasive media is being optimally accelerated through the nozzle and directed to the target surface. Accordingly, to clean large surface areas, it has been proposed to alter the blast nozzle shape, in particular, reconfigure the shape of the nozzle outlet so as to provide a larger hotspot and reduce cleaning time.
One such nozzle configuration is characterized as a fan nozzle in which the outlet section of the nozzle downstream of the venturi orifice diverges outwardly in two directions so as to provide the nozzle outlet with a fan-type shape. A fan nozzle has been developed specifically for blast cleaning with sodium bicarbonate. Thus, the present inventor of U.S. RE. Pat. No. 34,854, discloses a blast nozzle particularly useful in blasting with soft and friable media such as sodium carbonate and which nozzle can be characterized as a fan nozzle. The fan nozzle comprises a continuous longitudinal passageway comprising an inlet portion, which converges in a single planar axis, a rectangular venturi throat or orifice and an outlet portion, which diverges also in a single planar axis, which is perpendicular to the axis of convergence of the inlet portion. The converging passage in the inlet portion is formed by opposed modular triangular ramps, which can be removed and replaced with other ramps, which are longer or shorter so as to maximize the speed of the blast media and adjust the blast nozzle to readily accommodate different types of blast media operating conditions so as to maintain optimal productivity. The inlet portion of the fan nozzle is rigid, rectangular, and is sufficiently long that the length of the inlet portion of the blast nozzle is greater than twice the inside diameter of the blast nozzle inlet. The width of the orifice is the same as the diameter of the inlet. The longer convergence and avoidance of immediate expansion as the blast media/air stream enters the nozzle provides improved streamline flow, less turbulence and less mass loss in the individual abrasive particles. The outlet portion is also of modular construction comprising releasable attached upper and lower fan-shaped expansion sections which can be replaced to change the expansion ratio or angle of divergence of the nozzle and thus allows the nozzle to be adjusted to accommodate the specific media being used and changing on-site conditions.
The cross section of the outlet of the fan nozzle described in U.S. RE. Pat. No. 34,854 is rectangular. The rectangular cross section of the outlets of fan nozzles is typical of this blast nozzle configuration. Unfortunately, the rectangular cross section of the outlet provides a large internal nozzle surface area relative to the same cross sectional area of a round nozzle, thus, increasing drag on the expanding air and abrasive particles being directed through the nozzle. The increased drag reduces abrasive particle speed. Thus, while round nozzles produce a high intensity blast pattern in a small round area, the fan nozzle produces a lower intensity blast pattern over an elongated area. Depending on the application, a fan nozzle with lower particle speed can be more productive than a round nozzle with high particle speed depending upon the surface to be cleaned and the coating material to be removed.
Round nozzle geometry for producing the maximum nozzle efficiency, in particular, with softer blast media such as sodium bicarbonate has been defined as disclosed in aforementioned U.S. Pat. No. 5,484,325. However, little work has been focused on transferring the optimization of the round nozzle to optimize the fan nozzle efficiency and, in particular, to overcome the excessive drag which results utilizing the rectangular cross sectional dimensions typically used in fan nozzle configurations.
Accordingly, it is the object of the present invention to provide a novel fan nozzle design, as well to provide a fan nozzle geometry that provides for optimum blast cleaning efficiency when utilizing such fan nozzles for cleaning a targeted surface.
The round nozzle geometry yields a linear taper increasing uniformly in the X-Y and X-Z planes from the nozzle throat to the nozzle outlet with a uniform circular-cross section (Y-Z plane). The optimum linear taper is provided by the ratios with respect to outlet length to diameter of the orifice and outlet diameter relative to the orifice diameter as previously described in U.S. Pat. No. 5,484,325.
In accordance with the present invention, the optimum X-Y and X-Z plane geometries can be defined for a fan nozzle of specific throat diameter and outlet width by matching the cross sectional areas along the fan nozzle outlet section length to a round nozzle of the same throat diameter and outlet section length. Since the round nozzle represents the minimum internal surface area design for a particular size nozzle, a fan nozzle of the same size (length) will have a greater internal surface area and produce more drag. Increasing fan nozzle outlet width increases surface area and associated drag. Comparing the internal surface areas as a ratio between the same size round and fan nozzles can be used to predict performance or efficiency of a particular fan nozzle design. Thus, the present invention attempts to create a fan nozzle which has the improved productivity of a round nozzle of the same size by matching the cross sectional areas of the diverging outlet portion of the fan nozzle with the cross sectional areas of the outlet section of a round nozzle of optimum performance as previously described in the inventors aforementioned patent.
It has been found that the most efficient fan nozzle has an outlet portion having a cross-sectional shape (Y-Z plane) that is a classic ellipse, where the cross sectional area equals that of the corresponding position along the length of a round nozzle having the same size and configured for optimal performance as previously described. The outlet portion has a cross-section that is round at the nozzle throat and becomes progressively flatter in the Y dimension and uniformly wider in the Z dimension as the nozzle length increases in the X dimension to the nozzle outlet. The X-Z plane taper from the centerline of the nozzle is linear, increasing uniformly from the throat diameter to the nozzle outlet. The X-Y plane curve (from centerline) can be described by a polynomial equation.